The present invention relates generally to digital signal processing and signal conversion techniques. More particularly, the invention relates to a digital interface apparatus capable of providing high resolution and fast response time and suitable for interfacing sensors to systems requiring analog information. Although the invention is applicable to a wide variety of different types of sensors, it will be explained in the context of a noncontact displacement sensor.
One noncontact technique for sensing displacement or change in displacement involves using a resonant tank circuit in which an element of the tank circuit changes impedance in response to physical movement. The change in impedance results in a change in the resonant frequency of the tank circuit and that change in resonant frequency can be measured and related to the physical movement.
In a typical sensor embodiment is a rotary position sensor consisting of a pair of planar spiral coils placed on a stationary board with a pair of semicircular moving members located in a parallel plane above and below the coils. The moving members are mounted on a shaft extending between the two coils and perpendicular to the plane of the coils. See, for example, U.S. Pat. No. 4,644,570 which issued to Brosh and Landmann, entitled "Sensor Amplification and Enhancement Apparatus Using Digital Techniques." The position of the moving members varies the inductance of the coils in a complementary fashion. When the plate is rotated by the shaft in one direction, the inductance of one coil increases and the inductance of the other coil decreases in a related way. The coils are alternately connected to a capacitor and energized by an oscillator to establish a resonant tank circuit. The position of the tuning plate changes the coil inductance and thereby changes the resonant frequency. By measuring the resonant frequency when the tank circuit is in oscillation, the position of the tuning plate can be inferred.
By using dual complementary coils, and by multiplexing so that the two coils share the same tank circuit capacitor, many temperature effects and component drift errors are compensated for. The coils are alternately energized and de-energized, rather than simultaneously energized, to prevent electromagnetic cross-coupling interference between the two coils.
In the conventional arrangement described above, the respective coil resonant frequencies can be measured and a digital number or numbers representative of the resonant frequency obtained. The digital numbers obtained are then used as the basis for computing an integer which is used to modulate the duty cycle of a fixed frequency pulse train using the system clock. The duty cycle so modulated is thus indicative of resonant frequency and ultimately indicative position of the moving members. For example, in a digital system having 8 bits of resolution (256 possible numeric values) the number 128 is expressed as a 50% duty cycle. It will provide a pulse which is ON for 128 and OFF for 128 of the 256 clock pulse repetition rate. By comparison, the number 129 is expressed as a pulse of duty cycle slightly less than 50.4%, i.e., ON for 129 clock pulses and OFF for 127 clock pulses.
By integrating or averaging the variable duty cycle pulse train using a low pass filter, the variable duty cycle information is converted into an analog voltage. In this fashion the sensor provides an analog signal with voltage level indicating the tuning plate position.
The above duty cycle modulation approach has a serious limitation in applications requiring both high resolution and fast response time. For a system with a fixed main system clock rate, increasing the resolution causes the frequency of the variable duty cycle pulse train to be reduced because of the longer counts. In practice, the main system clock rate is fixed due to limited physical constraints of electronic circuitry. For example, present day CMOS devices in popular use constrain the clock rate to approximately 10 megahertz. Faster devices are anticipated, but clock rate will still be a limiting factor.
The base clock rate, in effect, dictates the frequency of the variable duty cycle pulse train. At a given base clock rate it takes twice as long to convey a pulse train with a repetition rate of 512 clock cycles as it does to convey a pulse train with a repetition rate of 256 clock cycles. Thus in improving resolution from an 8 bit system (256 states) to a 10 bit system (1,024 states) the pulse train must increase in period (or decrease in frequency) by a factor of 4. An increase in resolution from 8 bits to 12 bits would similarly result in a change by a factor of 16.
This imposed decrease in pulse train frequency (increase in period) has an impact upon the low pass filter used to integrate the pulse train and produce the analog voltage level. In order to filter out the significantly lower frequency cycle rate of the pulse train, the low pass filter must employ much longer time constants and hence have a much slower response time.
Resolution and response time thus are inversely proportional. Improving resolution degrades response time and vice versa. This has practical implications in using and designing sensors. For example, a foot pedal position sensor in a drive by wire automotive system may require both high resolution and fast response time. Present noncontact sensor technology is generally inadequate to meet such requirements.
The present invention overcomes the above dichotomy between resolution and response time exhibited by conventional sensor interface technology. The invention is able to simultaneously provide both high resolution and fast response time. The invention provides a digital interface which implements a modulated sequence or width-modulated duty cycle pulse train. The derived integer number indicative of the sensor position to be represented as a variable duty cycle pulse train is divided into a most significant part and a least significant part. The most significant part is converted into a sequence of a predetermined number of pulses in which the duty cycle of each pulse represents a value which corresponds to the most significant part and the repetition rate is that of the most significant part. The least significant part is used to selectively alter the width of selected ones of the pulses. The sequence of pulses, so altered, may be integrated over the sequence or averaged to extract an analog output indicative of the original integer number. The invention may also be used to represent rational numbers by operating on numerator and denominator separately in this fashion.
The apparatus and method of the invention permits high resolution data to be conveyed using a much higher frequency pulse train than is possible with the same clock rate using the conventional duty cycle modulation approach. The invention may be used as an interface for a sensor, resulting in a high resolution, fast-acting sensor not heretofore available. The invention may be implemented in hardware or software and is well suited to a microelectronic fabrication in which the entire circuit is masked onto a chip using ASIC techniques and employing SOT techniques for surface mounting directly to the sensor package.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.